1. Field of the Invention
The present invention relates to a display and a method for driving a display, and particularly to a display in which pixel circuits each including an electro-optical element are arranged in rows and columns (in a matrix), and a method for driving the display.
2. Description of Related Art
In recent years, development and commercialization of organic electro luminescence (EL) displays have been advanced. In the organic EL display, a large number of pixel circuits are arranged in a matrix, and each pixel circuit includes an organic EL element, which is a so-called current-driven light-emitting element of which emission luminance varies depending on the current value, as an electro-optical element. Since the organic EL element is a self-luminous element, the organic EL display has advantages such as high image visibility, no backlight, and high response speed over a liquid crystal display, which controls the intensity of light from a light source (backlight) by use of pixel circuits each including a liquid crystal cell.
As the driving system for the organic EL display, a simple-(passive) matrix system or an active-matrix system can be employed, similarly to the liquid crystal display. However, a display of the simple-matrix system involves a problem that it is difficult to realize a large-size and high-definition display and other problems although the configuration thereof is simple. For that reason, in recent years, development of displays of the active-matrix system has been actively promoted. In the active-matrix display, the current flowing through a light-emitting element is controlled by an active element such as an insulated gate field effect transistor (typically, thin film transistor; TFT) provided in the same pixel circuit as that including the light-emitting element.
If N-channel transistors can be used as the thin film transistors (hereinafter, referred to as TFTs) that are included in pixel circuits as active elements, an existing amorphous silicon (a-Si) process can be used in fabrication of the TFTs. The use of an a-Si process can reduce costs of the TFT substrate.
In general, the current-voltage (I-V) characteristic of an organic EL element deteriorates as the time passes (deteriorates with age). In a pixel circuit including N-channel TFTs, the source of the TFT for current-driving the organic EL element (hereinafter, referred to as a drive TFT) is connected to the organic EL element. Therefore, age deterioration of the I-V characteristic of the organic EL element leads to a change in the gate-source voltage Vgs of the drive TFT, which results in a change in the emission luminance of the organic EL element.
A more specific description will be made on this point. The source voltage of the drive TFT is determined depending on the operating point of the drive TFT and organic EL element. Deterioration of the I-V characteristic of the organic EL element varies the operating point of the drive TFT and organic EL element. Therefore, even when the same gate voltage is applied to the drive TFT, the source voltage of the drive TFT varies. Thus, the gate-source voltage Vgs of the drive TFT varies, and hence the value of the current flowing through the drive TFT varies. Accordingly, the value of the current flowing through the organic EL element also varies, which results in variation in the emission luminance of the organic EL element.
Furthermore, in addition to the age deterioration of the I-V characteristic of the organic EL element, the pixel circuit including N-channel TFTs involves a change in the threshold voltage Vth of the drive TFT with time and variation in the threshold voltage Vth from pixel to pixel. The difference in the threshold voltage Vth of the drive TFT leads to variation in the value of the current flowing through the drive TFT. Accordingly, even when the same gate voltage is applied to the drive TFT, the emission luminance of the organic EL element varies.
An existing related art employs a configuration in which each of pixel circuits is provided with a function to compensate variation in the characteristic of the organic EL element and a function to compensate variation in the threshold voltage Vth of the drive TFT so that the emission luminance of the organic EL element is not affected but kept constant even when the I-V characteristic of the organic EL element deteriorates with age and the threshold voltage Vth of the drive TFT changes over time (refer to e.g. Japanese Patent Laid-open No. 2004-361640). The related art according to this patent document will be described below.
FIG. 1 is a circuit diagram showing the configurations of an active-matrix display and pixel circuits used in the display according to the related art. The active-matrix display of the related art includes a pixel array 102 in which a large number of pixel circuits 101 each including a current-driven light-emitting element such as an organic EL element are arranged in a matrix. FIG. 1 shows the specific circuit configuration of certain one pixel circuit 101 for simplified illustration.
In the pixel array 102, scan lines 103, first and second drive lines 104 and 105, and auto-zero lines 106 are provided for the respective pixel circuits 101 on each row basis. Furthermore, data lines 107 are provided on each column basis. Arranged in the periphery of the pixel array 102 are a write scanning circuit 108 that drives the scan lines 103, first and second drive scanning circuits 109 and 110 that drive the first and second drive lines 104 and 105, respectively, an auto-zero circuit 111 that drives the auto-zero lines 106, and a data line drive circuit 112 that supplies the data lines 107 with data signals dependent upon luminance information.
The pixel circuit 101 includes, as its components, an organic EL element 201, a drive transistor 202, capacitors (storage capacitors) 203 and 204, a sampling transistor 205 and switching transistors 206 to 209. As the drive transistor 202, the sampling transistor 205 and the switching transistors 206 to 209, e.g. N-channel field effect TFTs are used. Hereinafter, the drive transistor 202, the sampling transistor 205 and the switching transistors 206 to 209 are referred to as a drive TFT 202, a sampling TFT 205 and switching TFTs 206 to 209, respectively.
The cathode electrode of the organic EL element 201 is coupled to a ground potential GND. The drive TFT 202 is a transistor that drives the organic EL element 201 to emit light, and the source thereof is connected to the anode electrode of the organic EL element 201, which leads to formation of a source follower circuit. The capacitor 203 is a storage capacitor. One electrode thereof is connected to the gate of the drive TFT 202, while the other electrode thereof is connected to a connecting node N101 between the source of the drive TFT 202 and the anode electrode of the organic EL element 201.
One terminal of the sampling TFT 205 is connected to the data line 107, the other terminal thereof is coupled to the gate of the drive TFT 202, and the gate thereof is connected to the scan line 103. One electrode of the capacitor 204 is connected to a node N104, while the other electrode thereof is connected to a connecting node N102 between the gate of the drive TFT 202 and one electrode of the capacitor 203. The drain of the switching TFT 206 is connected to the connecting node N101, and the source thereof is coupled to a supply potential Vss.
The drain of the switching TFT 207 is coupled to a positive supply potential Vcc, the source thereof is connected to the drain of the drive TFT 202, and the gate thereof is connected to the second drive line 105. One terminal of the switching TFT 208 is connected to a connecting node N103 between the drain of the drive TFT 202 and the source of the switching TFT 207, the other terminal thereof is connected to the connecting node N102, and the gate thereof is connected to the auto-zero line 106. One terminal of the switching TFT 209 is coupled to a predetermined potential Vofs, the other terminal thereof is connected to the node N104, and the gate thereof is connected to the auto-zero line 106.
In the following, a description will be made on the circuit operation of an active-matrix organic EL display in which the pixel circuits 101 each having the above-described configuration are two-dimensionally arranged in a matrix with reference to the timing chart of FIG. 2.
When the pixel circuit 101 on a certain row is driven, a write signal WS is supplied to the pixel circuit 101 from the write scanning circuit 108 via the scan line 103, and first and second drive signals DS1 and DS2 are supplied to the pixel circuit 101 from the first and second drive scanning circuits 109 and 110 via the first and second drive lines 104 and 105, respectively. Furthermore, an auto-zero signal AZ is supplied to the pixel circuit 101 from the auto-zero circuit 111 via the auto-zero line 106. FIG. 2 shows the timing relationship among these signals.
In a normal emission state, the write signal WS output from the write scanning circuit 108, the drive signal DS1 output from the first drive scanning circuit 109, and the auto-zero signal AZ output from the auto-zero circuit 111 are at the “L” level, while the drive signal DS2 output from the second drive scanning circuit 110 is at the “H” level. Therefore, the sampling TFT 205 and the switching TFTs 206, 208 and 209 are in the off-state, while the switching TFT 207 is in the on-state.
At this time, the drive TFT 202 operates as a constant current source since it is designed so as to operate in the saturation region. As a result, a constant current Ids expressed by Equation (1) is supplied from the drive TFT 202 to the organic EL element 201.Ids=(½)·μ(W/L)Cox(Vgs−|Vth|)2  (1)
In Equation (1), Vth is the threshold voltage of the drive TFT 202, μ is the carrier mobility, W is the channel width, L is the channel length, Cox is the gate capacitance per unit area, and Vgs is the gate-source voltage.
When the switching TFT 207 is in the on-state, both the drive signal DS1 output from the first drive scanning circuit 109 and the auto-zero signal AZ output from the auto-zero circuit 111 are turned to the “H” level, and hence the switching TFTs 206, 208 and 209 enter the on-state. Thus, the supply potential Vss is applied to the anode electrode of the organic EL element 201, while the supply potential Vcc is applied to the gate of the drive TFT 202.
At this time, if the supply potential Vss is lower than the sum between the cathode voltage Vcat of the organic EL element 201 (ground potential GND, in this example) and the threshold voltage Vthel of the organic EL element 201 (Vcat+Vthel), the organic EL element 201 becomes the non-emission state, which starts the non-emission period. The following description is based on an assumption that the relationship Vss≦Vcat+Vthel is satisfied and the supply potential Vss is at the GND level. When the non-emission period starts, since the switching TFTs 206 and 208 enter the on-state, the constant current Ids dependent upon the gate-source voltage Vgs flows through the path of Vcc→switching TFT 207→drive TFT 202→node N101→switching TFT 206→Vss.
Subsequently, the drive signal DS2 output from the second drive scanning circuit 110 is turned to the “L” level, so that the switching TFT 207 becomes the off-state and thus the operation time sequence enters a threshold cancel period for canceling (correcting) the threshold voltage Vth of the drive TFT 202. At this time, the drive TFT 202 operates in the saturation region since the gate and drain thereof are coupled to each other via the switching TFT 208. In addition, since the capacitors 203 and 204 are connected to the gate of the drive TFT 202 in parallel to each other, the gate-source voltage Vgs of the drive TFT 202 gradually decreases as the time passes.
After a certain period has passed, the gate-source voltage Vgs of the drive TFT 202 reaches the threshold voltage Vth of the drive TFT 202. At this time, a voltage of (Vofs−Vth) is charged to the capacitor 204, while a voltage of Vth is charged to the capacitor 203. Subsequently, when the sampling TFT 205 and the switching TFT 207 are in the off-state and the switching TFT 206 is in the on-state, the auto-zero signal AZ output from the auto-zero circuit 111 is changed from the H level to the “L” level. Thus, the switching TFTs 208 and 209 enter the off-state, which corresponds to the end of the threshold cancel period. At this time, the capacitor 204 holds the voltage of (Vofs−Vth), while the capacitor 203 holds the voltage of Vth.
Subsequently, when the sampling TFT 205 and the switching TFTs 207, 208 and 209 are in the off-state and the switching TFT 206 is in the on-state, the write signal WS output from the write scanning circuit 108 is turned to the “H” level, which starts a writing period. In the writing period, the sampling TFT 205 is in the on-state, which allows writing of an input signal voltage Vin supplied through the data line 107. Specifically, by turning on the sampling TFT 205, the input signal voltage Vin is loaded onto the connecting node N104 among one terminal of the TFT 205, one electrode of the capacitor 204 and the source of the TFT 209, so that a voltage variation amount ΔV at the connecting node N104 is coupled to the gate of the drive TFT 202 via the capacitor 204.
At this time, the gate voltage Vg of the drive TFT 202 is equal to the threshold voltage Vth, and the coupling amount ΔV is determined depending on the capacitance C1 of the capacitor 203, the capacitance C2 of the capacitor 204, and the parasitic capacitance C3 of the drive TFT 202 as expressed by Equation (2).ΔV={C2/(C1+C2+C3)}·(Vin−Vofs)  (2)
Therefore, if the capacitances C1 and C2 of the capacitors 203 and 204 are set sufficiently larger than the parasitic capacitance C3 of the drive TFT 202, the amount ΔV of the coupling to the gate of the drive TFT 202 is not affected by the threshold voltage Vth of the drive TFT 202 but determined depending only on the capacitances C1 and C2 of the capacitors 203 and 204.
When the write signal WS output from the write scanning circuit 108 is changed from the “H” level to the “L” level and hence the sampling TFT 205 is turned off, the period for writing the input signal voltage Vin ends. After the end of the writing period, when the sampling TFT 205 and the switching TFTs 208 and 209 are in the off-state, the drive signal DS1 output from the first drive scanning circuit 109 is switched to the “L” level, which turns off the switching TFT 206. Subsequently, the drive signal DS2 output from the second drive scanning circuit 110 is switched to the “H” level, which turns on the switching TFT 207.
The turning-on of the switching TFT 207 leads to a rise in the drain potential of the drive TFT 202 to the supply potential Vcc. Since the gate-source voltage Vgs of the drive TFT 202 is constant, the drive TFT 202 supplies the constant current Ids to the organic EL element 201. At this time, the potential at the connecting node N101 rises to a voltage Vx that permits the constant current Ids to flow through the organic EL element 201, which results in the light emission of the organic EL element 201.
Also in the pixel circuit 101 that executes the above-described series of operation, the I-V characteristic of the organic EL element 201 changes as the total emission period thereof becomes longer. Therefore, the potential at the connecting node N101 also changes.
However, since the gate-source voltage Vgs of the drive TFT 202 is kept at a constant value, the value of the current flowing through the organic EL element 201 does not change. Therefore, even when the I-V characteristic of the organic EL element 201 deteriorates, the constant current Ids invariably continues to flow, which causes no change in the emission luminance of the organic EL element 201. Furthermore, due to the operation of the switching TFT 208 in the threshold cancel period, the threshold voltage Vth of the drive TFT 202 can be cancelled, so that the constant current Ids that is not affected by variation in the threshold voltage Vth can be applied to the organic EL element 201, which allows achievement of high-quality images.
As described above, in the related art, each of the pixel circuits 101 is provided with a function to compensate variation in the I-V characteristic of the organic EL element 201 and a function to compensate variation in the threshold voltage Vth of the drive TFT 202. Thus, even when the I-V characteristic of the organic EL element 201 deteriorates with age and the threshold voltage Vth of the drive TFT 202 changes over time, the emission luminance of the organic EL element 201 can be kept constant without being affected by these changes.
However, the pixel circuit including N-channel TFTs involves variation in the carrier mobility μ of the drive TFT from pixel to pixel as well as the age deterioration of the I-V characteristic of the organic EL element and a change in the threshold voltage Vth of the drive TFT with time (variation from pixel to pixel). As is apparent from the aforementioned Equation (1), the difference in the mobility μ of the drive TFT among pixels causes variation in the current Ids flowing through the drive TFT from pixel to pixel, and therefore the emission luminance of the organic EL element varies from pixel to pixel, which results in a nonuniform image quality involving streaks and unevenness.